CeO2 nanoparticle-decorated reduced graphene oxide as an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions

Highly efficient bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential for the development of renewable energy technologies such as fuel cells and metal-air batteries. Herein, a ceria (CeO₂) – modified reduced graphene oxide (CeO₂/rGO) nanocomposite was fabricated via a facile yet cost-effective method under a mild condition. The prepared CeO₂/rGO nanocomposite showed remarkable catalytic activity, high tolerance to methanol and durability toward ORR in alkaline media. Meanwhile, the catalyst also displayed remarkable activity for the OER with more negative onset potential and higher current compared with commercial Pt/C catalyst. The high oxygen reaction activity of the catalyst could contribute to synergistic effect of the combination of the oxygen vacancies of CeO₂ and excellent electronic conductivity of rGO. The results suggested that the CeO₂/rGO nanocomposite has potential advantages as a bifunctional electrocatalyst in the practical applications.     

Investigation of the oxygen reduction reaction on the carbon electrodes loaded with MnO2 catalyst

The oxygen reduction reaction has been studied on gas diffusion electrodes made with various activated carbon materials and on the edge/basal orientations of pyrolitic graphite. A MnO₂ catalyst was loaded on all carbon surfaces. The MnO₂ catalyst demonstrated significant catalytic activity for the oxygen reduction reaction. The specific catalytic activity was found to relate to the concentration of the edge orientation of carbon materials loaded with MnO₂ catalyst. The higher the percentage of edge orientations, the higher the specific catalytic activity would be. MnO₂ may not participate in the reduction of O₂, but catalyze the disproportionation of HO₂⁻.     

Facile preparation of biomass-derived bifunctional electrocatalysts for oxygen reduction and evolution reactions

The development of inexpensive and efficient bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is still remained a challenge in wide range of renewable energy technologies. Herein, biomass-derived nitrogen self-doped porous carbon nanosheets (NPCNS) are produced by a facile and green pyrolysis of Euonymus japonicus leaves at controlled temperature and then the nitric acid pickling was carried out to remove the excess metal ingredients. The obtained NPCNS exhibits a hierarchically porous distribution, high BET surface area and uniform nitrogen doping. Electrochemical measurements show that the NPCNS possess a high electrocatalytic activity for both ORR and OER. Among these NPCNS catalysts, the sample carbonized at 900 °C (NPCNS-900) with the highest concentration of pyridinic nitrogen shows the best ORR and OER activity. According to our DFT calculations, the high content of pyridinic nitrogen with the moderate O and OH adsorption energies among the three types of nitrogen should be the critical factor for the efficient catalytic performance of NPCNS-900 toward ORR and OER. This work demonstrates that the facile prepared NPCNS-900 is a potential candidate material with excellent performance in electrocatalytic applications such as fuel cells or metal-air batteries.     

Preparation of silver-modified La0.6Ca0.4CoO3 binary electrocatalyst for bi-functional air electrodes in alkaline medium

Silver-modified La_0.6Ca_0.4CoO₃ composites for molecular oxygen reduction and evolution reaction are prepared by a chemical reduction process using N₂H₄ as the reducing agent at room temperature. The La_0.6Ca_0.4CoO₃ catalysts are modified with silver content that vary from 0.3 to 30 wt.% without damaging their microstructure. The electrochemical behavior of La_0.6Ca_0.4CoO₃ catalysts with different silver loadings is studied on classical bilayer gas diffusion electrodes. The electrocatalytic properties of these composites are evaluated by polarization curves and electrochemical impedance spectroscopy in alkaline electrolyte. The silver loading is found to have a significant impact on the electrode performances, which facilitate or block the electrochemical processes of the gas diffusion electrodes. The binary catalyst electrodes exhibit higher electrocatalytic activities than that of the electrodes with only La_0.6Ca_0.4CoO₃ as the catalyst. In this paper, the best performance was achieved when the silver loading was 3.0 wt.%.     

Linear sweep voltametry studies on oxygen reduction of some oxides in alkaline electrolytes

The study uses linear sweep voltametry (LSV) to observe the efficiency of oxygen reduction on some oxides and their mixtures in 6 M KOH at 25 °C. The investigated materials are Ag₂O, MnO₂, Sm₂O₃, Dy₂O₃ and NdO₂. The electrocatalytic oxygen reduction reactions (ORR) on Teflon-bonded, oxide + graphite electrodes are studied. The oxygen reduction potentials for electrodes containing these materials as catalyst are seen as −60.67, −270.31, −111, −159.58 and −130.24 mV, respectively. Mixture combinations of these oxides give a higher ORR peak current thereby showing evidence of synergetic effect. Air–MH cells using some of the above investigated oxides as catalyst for air electrode are constructed and studied. Best performance is obtained with silver oxide. The LSV findings are in accordance with air–MH cell charge/discharge experiments and for best performance prefer shift of the ORR onset potential to more positive positions.     

Electrodeposited Ni–Co-oxide electrodes:characterization and kinetics of the oxygen evolution reaction

In this paper we present results on the characterization of Ni–Co-oxide electrodes, prepared by anodic deposition from Co(NO₃)₂ aqueous solutions on Ni substrates. The kinetics and mechanism of oxygen evolution was analysed. Tafel slopes close to 40 mV per decade were measured. The reaction order with respect to [OH⁻] was found to be approximately 2 at 25°C. A possible mechanism for oxygen evolution on these electrodes is presented, which accounts for the values of the kinetic parameters experimentally obtained.     

A facile method for reduced CoFe2O4 nanosheets with rich oxygen vacancies for efficient oxygen evolution reaction

The efficiency of electrochemical water splitting is greatly hindered by the thermodynamic uphill reaction of oxygen evolution reaction (OER). Thus, it is important to synthesize an active OER electrocatalysts with abundant active sites, favorable conductivity and good durability. Herein, a facile reduction method using NaBH₄ as readily available reductant has been developed to fabricate the reduced CoFe₂O₄ nanosheets (NS). The obtained reduced CoFe₂O₄ NS are rich in oxygen deficient sites, leading to more active sites as well as the enhanced conductivity than the pristine CoFe₂O₄ hollow nanosphere, which reaches the current density of 10 mA cm^?2 at the overpotential of 320 mV in 1 M KOH. Meanwhile, CoFe₂O₄ samples with three different morphology nanostructures including hollow nanospheres, bulk and nanoparticles have been provided to study the effect of different morphology on NaBH₄ reduction efficiency. As expected, after NaBH₄ reduction, CoFe₂O₄ hollow nanosphere with relatively higher surface area exhibits most obvious improvement for OER activity and also its corresponding reduced CoFe₂O₄ NS showed best OER performance than the reduced CoFe₂O₄ bulk as well as the reduced CoFe₂O₄ nanoparticles, implying the hollow nanospheres feature more accessible surface area than bulk and nanoparticles samples, thus greatly facilitate efficiency of NaBH₄ reduction treatment.     

Three-dimensional hierarchically porous iridium oxide-nitrogen doped carbon hybrid: An efficient bifunctional catalyst for oxygen evolution and hydrogen evolution reaction in acid

Active and durable acid medium electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are of critical importance for the development of proton exchange membrane (PEM) water electrolyser or Fuel cells. Herein, we report a facile method for the synthesis of 3D-hierarchical porous iridium oxide/N-doped carbon hybrid (3D-IrO₂/N@C) and its superior OER and HER activity in acid. In 0.5 M HClO₄, this catalyst exhibited remarkable activity towards OER with a low overpotential of 280 mV at 10 mA/cm² current density, a low Tafel slope of 45 mV/dec and ∼98% faradaic efficiency. The mass activity (MA) and turnover frequency (TOF) are found to be 833 mA/mg and 0.432 s⁻¹ at overpotential of 350 mV which are ∼32 times higher than commercial (comm.) IrO₂. The HER performance of this 3D-IrO₂/N@C is comparable with comm. Pt/C catalyst in acid. This 3D-IrO₂/N@C catalyst requires only 35 mV overpotential to reach a current density 10 mA/cm² with Tafel slope 31 mV/dec. Most importantly, chronoamperometric stability test confirmed superior stability of this catalyst towards HER and OER in acid. This 3D-IrO₂/N@C catalyst was applied as both cathode and anode for over-all water splitting and required only 1.55 V overpotential to achieve a current density of 10 mA/cm² in acid. The outstanding activity of the 3D-IrO₂/N@C catalyst can be attributed to a unique hierarchical porous network, high surface area, higher electron and mass transportation, synergistic interaction between IrO₂ and carbon support.     

NiFe layered double hydroxide as an efficient bifunctional catalyst for electrosynthesis of hydrogen peroxide and oxygen

Two electron oxygen reduction reaction to produce hydrogen peroxide (H₂O₂) is a promising alternative technique to the multistep and high energy consumption anthraquinone process. Herein, Ni–Fe layered double hydroxide (NiFe-LDH) has been firstly demonstrated as an efficient bifunctional catalyst to prepare H₂O₂ by electrochemical oxygen reduction (2e^? ORR) and oxygen evolution reaction (OER). Significantly, the NiFe-LDH catalyst possesses a high faraday efficiency of 88.75% for H₂O₂ preparation in alkaline media. Moreover, the NiFe-LDH catalyst exhibits excellent OER electrocatalytic property with small overpotential of 210 mV at 10 mA cm^?2 and high stability in 1 M KOH solution. On this basis, a new reactor has been designed to electrolyze oxygen and generate hydrogen peroxide. Under the ultra-low cell voltage of 1 V, the H₂O₂ yield reaches to 47.62 mmol g_cat^?1 h^?1. In order to evaluate the application potential of the bifunctional NiFe-LDH catalyst for H₂O₂ preparation, a 1.5 V dry battery has been used as the power supply, and the output of H₂O₂ reaches to 83.90 mmol g_cat^?1 h^?1. The excellent electrocatalytic properties of 2e^? ORR and OER make NiFe-LDH a promising bifunctional electrocatalyst for future commercialization. Moreover, the well-designed 2e^? ORR-OER reactor provides a new strategy for portable production of H₂O₂.     

Influence of air electrode electrocatalysts on performance of air-MH cells

Ni-MH battery system has been in limelight recently because of its inherent advantages like high-energy density, eco-friendly nature, devoid of memory effect, etc. Replacement of the heavy nickel oxide electrode with lighter air electrode is expected to improve its energy density further by 20% and also to bring down the cost. Hence some studies have appeared in literature on the development of air-MH system. But the main problem is to have an adequate bifunctional electrode with suitable electrocatalyst. Several materials are available for use in air electrodes. A detailed study is required to identify the best catalytic material and optimize the battery activity. Hence, air-MH cells using different oxides like _Ag2O${\mathrm{Ag}}_2\mathrm{O}$, _LaMnO3${\mathrm{LaMnO}}_3$ and _{La0.65Sr0.30MnO3}${\mathrm{La}}_{0.65}{\mathrm{Sr}}_{0.30}{\mathrm{MnO}}_3$ as catalysts for air electrode have been investigated in the present study with _{MmNi3.5Co0.8Mn0.4Al0.3}${\mathrm{MmNi}}_{3.5}{\mathrm{Co}}_{0.8}{\mathrm{Mn}}_{0.4}{\mathrm{Al}}_{0.3}$ metal hydride negative electrode. Life cycling along with charge and discharge characteristics was studied in detail. The air-MH cells assembled with _Ag2O${\mathrm{Ag}}_2\mathrm{O}$ and _LaMnO3${\mathrm{LaMnO}}_3$ as catalysts in the air electrode gave encouraging performance. _LaMnO3${\mathrm{LaMnO}}_3$ when incorporated as electrocatalyst delivered stable cycle life whereas incorporation of Sr resulted in inferior performance in the studied composition range.     

Reversible air electrodes integrated with an anion-exchange membrane for secondary air batteries

Reversible air electrodes integrated with a polymer electrolyte membrane have been proposed for use in rechargeable metal-air batteries or unitized regenerative fuel cells to reduce the impact of atmospheric carbon dioxide. Reversible air electrodes were prepared with an anion-exchange membrane (AEM) as a polymer electrolyte membrane and platinum-based catalysts. The AEM at the interface between the alkaline electrolyte and the air electrode layer plays major roles in AEM-type air electrodes as follows: it blocks (a) the permeation of cations in the alkaline electrolyte into the air electrode layer to prevent carbonate precipitation, (b) penetration of the alkaline solution itself, and (c) neutralization of the alkaline electrolyte by carbon dioxide, all of which prevent performance degradation of oxygen reactions. Catalysts for decreasing the overvoltage of oxygen reactions were also investigated with the AEM-type air electrode, and the overall efficiency was improved due to a remarkable decrease in the potential for the oxygen evolution reaction with Pt-Ir catalysts.     

Improving catalytic activity of layered lithium transition metal oxides for oxygen electrode in metal-air batteries

Lithium transition metal oxide has superior performance for oxygen evolution reaction (OER), while its activity for catalyzing oxygen reduction reaction (ORR) is too low to meet the demand of practical applications. Herein, the NCM-based (NCM, LiNi_1/3Co_1/3Mn_1/3O₂) composite materials are prepared through the two steps method. The NCM-2 (Mn₂O₃/LiNi_1/3Co_1/3Mn_1/3O₂) hybrid material demonstrates excellent ORR catalytic property and high OER catalytic performance, as well as the superior stability. Besides, with NCM-2 hybrid materials as catalysts of air cathode, the Al-air battery and Zn-air battery both exhibit higher power density. Therefore, based on results of Brunauer-Emmett-Teller and O₂ temperature programmed desorption analysis, the improved catalytic performance ascribed to large specific surface area, pore structure and enhanced oxygen adsorption ability. In this work, the catalytic activity of lithium transition metal oxide has been improved, and a new method was provided to synthesize bifunctional catalysts for metal-air batteries.     

Facile synthesis of BSCF perovskite oxide as an efficient bifunctional oxygen electrocatalyst

We present a facile way to synthesize BSCF by using glycine-nitrate auto-combustion followed by annealing at different conditions, which work as high-performance bifunctional electrocatalyst for oxygen evolution (OER) as well as oxygen reduction (ORR) reactions in alkaline solution with comparatively better efficiency for OER. Annealing condition plays an important role towards catalytic performance due to morphological control and surface composition. Although, there is no significant change in onset potentials but these catalysts afford a current density >10 mA cm^?2 at the potential of 1.65 V for oxygen evolution reaction and a current density >2.5 mA cm^?2 at the potential of 0.009 V for oxygen reduction reaction with respect to RHE in 0.1 M KOH. The underlying mechanism for ORR and OER as well as catalytic activity differences were understood with the help of different analytical characterization techniques.     

Uniform cobalt grafted on vanadium nitride as a high efficient oxygen evolution reaction catalyst

The rational design of highly effective and low-cost catalysts for oxygen evolution reaction (OER) is of prime importance for the development of water splitting. However, the activity of electrocatalysts still needs enhancement to satisfy the practical application. Herein, we report Co nanoparticles grafted on vanadium nitride (VN) surface via in situ phase separation method by nitriding Co₂V₂O₇ precursor. Benefiting the advantages of abundant active sites of Co, high conductivity and corrosion resistance of VN, the Co/VN achieves incredibly high activity and durability for OER with a low overpotential of 320 mV at a current density of 10 mV cm^?2 with a small Tafel slope of 50.4 mV dec^?1 and long-term stability. In addition, the in situ Raman further reveals the synergistic effect of Co and VN. Significantly, this study may enrich our knowledge and it can be extended to prepare other interconnected framework structures for the development of OER catalysts.     

N-doped graphene as a bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions in an alkaline electrolyte

In this work, a nitrogen-doped graphene (NG) catalyst was prepared using a hydrothermal method with ammonia as the nitrogen precursor, which was followed by a freeze-dry process. The catalyst was characterized using X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscope, and X-ray photoelectron spectroscopy. The bifunctional catalytic activities for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) were investigated using cyclic voltammetry in an alkaline electrolyte. The results indicate that nitrogen is successfully doped in the NG catalyst, and the catalyst has a loose structure that was produced during the freeze-dry process. The catalyst exhibits an excellent ORR activity with an onset potential of −0.08 V and a high OER activity with an obvious OER current at 0.7 V. The rotating-disk-electrode test results indicate that the ORR process catalyzed by the NG catalyst involves a mix of the two-electron and four-electron transfer pathways. This work preliminarily explores the bifunctional catalytic properties for the ORR and the OER of nitrogen-doped graphene materials in alkaline electrolyte.     

Oxygen evolution reaction on Ni-substituted Co3O4 nanowire array electrodes

Nanowire arrays of mixed oxides of Co and Ni freely standing on Ni foam are prepared by a template-free growth method. The effects of Ni content on the morphology, structure and catalyst performance for oxygen evolution reaction are investigated by scanning electron microscopy, X-ray diffraction spectroscopy and electrochemical techniques including cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy. A transformation from nanowire arrays to nanoplate arrays is found with the increase of the atomic ratio of Ni to Co in the preparation solution. The Ni_xCo_3−xO₄ electrode obtained at 1:1 of Ni:Co in the preparation solution exhibits nanowire array structure and has better catalytic performance for oxygen evolution reaction than other Ni_xCo_3−xO₄ and Co₃O₄ electrodes. The catalytic activities of the Ni_xCo_3−xO₄ and Co₃O₄ electrodes are correlated with their surface roughness. Superior stability of the Ni_xCo_3−xO₄ nanowire array electrode is demonstrated by a chronopotentiometric test. The reaction orders with respect to OH⁻ on the Ni_xCo_3−xO₄ electrode are close to 2 and 1 at low and high overpotentials, respectively.     

Highly active and stable bi-functional NiCoO2 catalyst for oxygen reduction and oxygen evolution reactions in alkaline medium

Single step solution combustion technique was used to synthesize NiO, Co₃O₄ and NiCoO₂ mixed metal oxide with good crystallinity and uniform properties. XRD spectrum indicates the existence of cubic NiCoO₂ phase without any impurities. SEM results indicate the presence of porous structures in all the three cases, a typical characteristic of combustion synthesized samples, which is due to the evolution of gases during the synthesis process. TEM along with the phase mapping shows the presence of well dispersed elements Ni, Co and O throughout the sample. All the three catalysts were evaluated for their bifunctionality towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline medium. NiCoO₂ shows the highest number of electron transfer in the overall reaction mechanism with the maximum kinetic current density of 12.3 mA/cm². The kinetics of NiCoO₂ towards ORR and OER was analyzed using Tafel plot and compared with the mono-metal oxides. The catalytic stability was evaluated for 24 h using continuous chronoamperometric (CA) runs, where NiCoO₂ shows exceptionally stable performance without any significant decay in current. The highest activity of NiCoO₂ could be due to the presence of higher oxidation states of Ni and Co and because of the existence of the oxygen defects acting as active sites for the oxygen adsorption/desorption during the electrocatalytic reactions. Based on the activity and stability trends, NiCoO₂ is found to be a promising bifunctional oxygen electrocatalyst for long-term applications.     

Development of NiO/Co3O4 nanohybrids catalyst with oxygen vacancy for oxygen evolution reaction enhancement in alkaline solution

The oxygen evolution reaction (OER) is a significant reaction in water splitting and energy conversion. However, high price and sluggish kinetics catalysts prevent commercial applications. Generally, noble metals (e.g., iridium and ruthenium), which are expensive and unstable, have been used as catalysts for OER because of their high electrocatalytic activity. In this study, we report a high-performance OER catalyst with oxygen vacancies comprising NiO/Co₃O₄ nanohybrids. For OER, the NiO/Co₃O₄ heterostructure show good electrocatalytic performance with a low overpotential of 330 mV. This is higher than those of NiO, Co₃O₄, and benchmark IrO₂ candidates at current density of 10 mA cm^?2. Furthermore, the NiO/Co₃O₄ nanohybrids show long-term electrochemical stability for 10 h. The present research results show that NiO/Co₃O₄ heterostructure is an excellent electrocatalyst for OER.     

Y and Fe co-doped LaNiO3 perovskite as a novel bifunctional electrocatalyst for rechargeable zinc-air batteries

Perovskite oxides are widely regarded as the promising air electrode catalytic materials for zinc-air batteries (ZABs). In the present work, A-site Y and B-site Fe co-doped La_0.85Y_0.15Ni_0.7Fe_0.3O₃ perovskite catalyst was prepared by self-propagating high-temperature synthesis, and this material was evaluated as a bifunctional electrocatalyst for ZABs. The effect of co-doping on crystal structure and reaction activities, which can promote oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), was investigated. Results show that Y and Fe co-doping substantially improved the ORR and OER of LaNiO₃. In comparison with LaNiO₃, the ORR performance of La_0.85Y_0.15Ni_0.7Fe_0.3O₃ exhibited a higher limiting current density (3.8 mA cm^?2 at 0.4 V vs. RHE) and more positive onset potential (0.75 V vs. RHE) at 1600 rpm. It also had an excellent OER performance of 1.74 V vs. RHE at 10 mA cm^?2. When La_0.85Y_0.15Ni_0.7Fe_0.3O₃ was used as an air electrode catalyst for ZABs, it exhibited a high power density of 93.6 mW cm^?2, which increased by 84.8% compared with that of LaNiO₃. Moreover, the full cell with La_0.85Y_0.15Ni_0.7Fe_0.3O₃ air electrode catalyst was operated for more than 80 h, maintaining good stability. Therefore, La_0.85Y_0.15Ni_0.7Fe_0.3O₃ can be used as a promising bifunctional air electrode catalyst for ZABs. The characterization analysis reveals that A-site Y and B-site Fe co-doped catalyst transforms crystal structure from trigonal system to cubic system, retain the valence state of Ni³⁺ and increases the contents of O₂^2?/O^?, and these properties are more conducive for LaNiO₃ catalysis.     

Single-site and nanosized Fe-Co electrocatalysts for oxygen reduction: Synthesis, characterization and catalytic performance

The impregnation of Ketjen Black (C) with iron and cobalt phthalocyanines (MPc) taken one by one or as a 1:1 stoichiometric mixture, followed by heat treatment at 600 °C under inert atmosphere, gave materials containing arrays of single metal ions coordinated by four nitrogen atoms (M-N₄ units). Increasing the pyrolysis temperature to 800° resulted in the formation of carbon-supported, nanosized metal particles. A key role of the carbon support in determining the material structure at either temperature investigated was demonstrated by TPD, EXAFS, XANES and XRPD studies. These also showed that a Fe-Co alloy is obtained at 800 °C when the impregnation of Ketjen Black involves a mixture of FePc and CoPc. Electrodes coated with the different Fe, Co and Fe-Co materials, containing ca. 3 wt% metal loadings, were scrutinized for the oxygen reduction reaction (ORR) in alkaline media by linear sweep voltammetry. For comparative purposes, two Pt electrocatalysts containing 3 and 20 wt% metal were investigated. The electrochemical activity of all materials was analyzed by Tafel and Koutecky-Levich plots as well as chronopotentiometry. The Fe-containing electrocatalysts have been found to be highly active for the ORR in alkaline media with convective limiting currents as high as 600 A g Fe⁻¹ at room temperature and onset potentials as high as 1.02 V vs. RHE. It has been found that (i) the ORR mass activity of the Pc-derived electrocatalysts is superior to that of the Pt catalysts investigated; (ii) the activity of FePc and FePc-CoPc/C, heat treated at either 600 or 800 °C, is superior to that of the corresponding Co materials; (iii) the electrocatalysts obtained at 600 °C are fairly more active than those obtained at 800 °C.